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We used a change point analysis on a late Pleistocene–Holocene lake-sediment record from the Chew Bahir basin in the southern Ethiopian Rift to determine the amplitude and duration of past climate transitions. The most dramatic changes occurred over 240 yr (from ~15,700 to 15,460 yr) during the onset of the African Humid Period (AHP), and over 990 yr (from ~4875 to 3885 yr) during its protracted termination. The AHP was interrupted by a distinct dry period coinciding with the high-latitude Younger Dryas stadial, which had an abrupt onset (less than ~100 yr) at ~13,260 yr and lasted until ~11,730 yr. Wet-dry-wet transitions prior to the AHP may reflect the high-latitude Dansgaard-Oeschger cycles, as indicated by cross-correlation of the potassium record with the NorthGRIP ice core record between ~45–20 ka. These findings may contribute to the debates regarding the amplitude, and duration and mechanisms of past climate transitions, and their possible influence on the development of early modern human cultures. [ABSTRACT FROM AUTHOR]

We describe the career of John Birks as a pioneering scientist who has, over a career spanning five decades, transformed palaeoecology from a largely descriptive to a rigorous quantitative science relevant to contemporary questions in ecology and environmental change. We review his influence on students and colleagues not only at Cambridge and Bergen Universities, his places of primary employment, but also on individuals and research groups in Europe and North America. We also introduce the collection of papers that we have assembled in his honour. The papers are written by his former students and close colleagues and span many of the areas of palaeoecology to which John himself has made major contributions. These include the relationship between ecology and palaeoecology, late-glacial and Holocene palaeoecology, ecological succession, climate change and vegetation history, the role of palaeoecological techniques in reconstructing and understanding the impact of human activity on terrestrial and freshwater ecosystems and numerical analysis of multivariate palaeoecological data. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic and geochemical core data spanning the last 17,000years are correlated with new seismic stratigraphy from Lake Tana, Ethiopia, to infer past lake-level change and hence effective precipitation. The data confirm that low lake-level coincides with Heinrich Event 1 (H1) in the North Atlantic, as previously shown from diatom and pollen evidence (Lamb et al., 2007). The lake deepened at 15.3calkyr BP and abruptly returned to freshwater conditions, when the lake overflowed into the Blue Nile. Low runoff and lake levels and therefore rainfall are inferred between 13.0 and 12.5calkyr BP and may represent southerly suppression of the ITCZ and the associated monsoon front at the time of the Younger Dryas. Two drought episodes occurred at 8.4 and 7.5calkyr BP, and are also interpreted as a southward shift in the monsoon front. The first of these events appears to have preceded and been more significant than the 8.2calkyr BP. Precipitation declined after 6.8calkyr BP, although we do not see an abrupt end to the African Humid Period. This period culminated in a dry episode at ~4.2calkyr BP, supporting the view that reduced Nile flow was a contributing factor to the demise of the Egyptian Old Kingdom. [Copyright &y& Elsevier]

Pollen data from a radiocarbon-dated lake-sediment core from the Middle Atlas of Morocco show that forests of evergreen and winter-deciduous oaks (Quercus rotundifolia, Q. canariensis), present from the start of the Holocene, were invaded at 6200 BP by Cedrus atlantica in low numbers. Two thousand years later, after a short period characterized by higher deciduous oak frequency, Cedrus increased to its present abundance. These changes may have been in response to increasing effective moisture as summer temperatures decreased with declining northern-hemisphere seasonality, under the influence of precessional forcing. Palaeolimnological data from the same core show that the lake level fell sharply in five 200-400 yr-long arid intervals, but the pollen data show little or no evidence of a vegetational response. This suggests that summer (growing-season) rainfall remained adequate during the dry intervals, whereas depletion of the groundwater aquifer was the result of reduced winter precipitation. Anthropogenic exploitation since c. 1300 BP has had a greater effect on the forest ecosystem than any of the Holocene arid intervals. [ABSTRACT FROM PUBLISHER]

Abstract Multi-proxy analyses of an 8 m sediment core from Lake Hayk, a closed, freshwater lake in the north-central highlands of Ethiopia, provide a record of changing lake level and inferred regional climatic change for the last 15.6 cal ka years. Between ca. 15.6–15.2 cal ka BP, a lowstand was synchronous with Heinrich Event 1 and an intense drought across Eastern Africa. At ca. 15.2 cal ka BP a lake began to develop at the core site in response to wetter conditions, at the onset of the African Humid Period (AHP). However, in contrast to other lakes in eastern Africa, Hayk lake level fell around ca. 14.8 cal ka BP, indicating a climate shift towards aridity. The lake began filling again at ca. 12.3 cal ka BP and reached maximum water depth between ca. 12.0–10.0 cal ka BP. Lake level declined slowly during the Holocene, culminating in the termination of the AHP at Hayk between ca. 5.2–4.6 cal ka BP. In the late Holocene, ca. 2.2–1.3 cal ka BP, Lake Hayk was again deep and fresh with some evidence of short-term lake level variability. The palaeo-record from Lake Hayk indicates that while it experienced, to a broad degree, the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from eastern Africa, there are offsets in timing and rate of response. These differences reflect chronological discrepancies between records, as well as the varying climate sensitivities and site-specific factors of individual lake basins. This record highlights the different responses by lakes in a climatically vulnerable area of Ethiopia. Highlights • Discrepancies in the timing of the Younger Dryas Stadial proper. • No evidence for the catastrophic shift in climate centred at 8.2 ka. • Diatom evidence suggesting relatively rapid AHP termination. • High-resolution evidence of millennial to multi-decadal variability at Lake Hayk. • Regional discrepancies in the timing and expression of climatic events. [ABSTRACT FROM AUTHOR]

The development and application of lacustrine paleotemperature proxies based on microbial membrane lipid structures, including the TEX 86 and branched glycerol dialkyl glycerol tetraether (brGDGT) paleothermometers, have greatly advanced our understanding of the late-glacial and postglacial temperature history of Africa. However, the currently available records are from equatorial and southern hemisphere sites, limiting our understanding of the spatial patterns of temperature change. Here we use the brGDGT paleotemperature proxy to reconstruct Late Pleistocene and Holocene temperatures from Lake Tana, Ethiopia (12°N, 37°E). Following the termination of Heinrich Stadial 1 at ~ 15 ka, Lake Tana experienced a 3.7 °C oscillation over 1.2 ky. Temperatures then increased abruptly by nearly 7 °C between 13.8 and 13.0 ka, followed by a slow warming trend that peaked during the mid Holocene. Temperatures subsequently cooled from ~ 6 ka to ~ 0.4 ka. These data indicate that temperature at Lake Tana was sensitive to climate changes caused by variations in the Atlantic Meridional Overturning Circulation during the Late Pleistocene, as well as to regional hydroclimatic changes and reorganizations of the monsoons. Our record suggests that late-glacial temperature changes in northeast Africa were linked to high-latitude northern hemispheric climate processes, but that subsequent post-glacial temperature variations were strongly influenced by tropical hydrology. [ABSTRACT FROM AUTHOR]